Memory device with switching glass layer

ABSTRACT

A memory device, such as a PCRAM, including a chalcogenide glass backbone material with germanium telluride glass and methods of forming such a memory device.

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/916,421, filed Aug. 12, 2004, now U.S. Pat. No. 7,190,048entitled PCRAM Device With Switching Glass Layer, and is also acontinuation-in-part of U.S. patent application Ser. No. 10/893,299,filed Jul. 19, 2004, entitled Resistance Variable Memory Device andMethod of Fabrication.

FIELD OF THE INVENTION

The invention relates to the field of random access memory (RAM) devicesformed using a resistance variable material.

BACKGROUND

Resistance variable memory elements, which include ProgrammableConductive Random Access Memory (PCRAM) elements, have been investigatedfor suitability as semi-volatile and non-volatile random access memorydevices. In a typical PCRAM device, the resistance of a chalcogenideglass backbone can be programmed to stable lower conductivity (i.e.,higher resistance) and higher conductivity (i.e., lower resistance)states. An unprogrammed PCRAM device is normally in a lowerconductivity, higher resistance state.

A conditioning operation forms a conducting channel of ametal-chalcogenide in the PCRAM device, which supports a conductivepathway for altering the conductivity/resistivity state of the device.The conducting channel remains in the glass backbone even after thedevice is erased. After the conditioning operation, a write operationwill program the PCRAM device to a higher conductivity state, in whichmetal ions accumulate along the conducting channel(s). The PCRAM devicemay be read by applying a voltage of a lesser magnitude than required toprogram it; the current or resistance across the memory device is sensedas higher or lower to define the logic “one” and “zero” states. ThePCRAM may be erased by applying a reverse voltage (opposite bias)relative to the write voltage, which disrupts the conductive pathway,but typically leaves the conducting channel intact. In this way, such adevice can function as a variable resistance memory having at least twoconductivity states, which can define two respective logic states, i.e.,at least a bit of data.

One exemplary PCRAM device uses a germanium selenide (i.e., Ge_(x)Se_(100-x)) chalcogenide glass as a backbone. The germanium selenide glasshas, in the prior art, incorporated silver (Ag) by (photo or thermal)doping or co-deposition. Other exemplary PCRAM devices have done awaywith such doping or co-deposition by incorporating a metal-chalcogenidematerial as a layer of silver selenide (e.g., Ag₂Se), silver sulfide(AgS), or tin selenide (SnSe) in combination with a metal layer,proximate a chalcogenide glass layer, which during conditioning of thePCRAM provides material to form a conducting channel and a conductivepathway in the glass backbone.

Extensive research has been conducted to determine suitable materialsand stoichiometries thereof for the glass backbone in PCRAM devices.Germanium selenide having a stoichiomety of about Ge₄₀Se₆₀ (i.e.,Ge₂Se₃), as opposed to Ge₂₃Se₇₇ or Ge₃₀Se₇₀, for example, has been foundto function well for this purpose. A glass backbone of Ge₄₀Se₆₀, with anaccompanying metal-chalcogenide (e.g., typically silver selenide) layer,enables a conducting channel to be formed in the glass backbone duringconditioning, which can thereafter be programmed to form a conductivepathway. The metal-chalcogenide is incorporated into chalcogenide glasslayer at the conditioning step. Specifically, the conditioning stepcomprises applying a potential (about 0.20 V) across the memory elementstructure of the device such that metal-chalcogenide material isincorporated into the chalcogenide glass layer, thereby forming aconducting channel within the chalcogenide glass layer. It is theorizedthat Ag₂Se is incorporated onto the glass backbone at Ge—Ge sites vianew Ge—Se bonds, which allows silver (Ag) migration into and out of theconducting channel during programming. Movement of metal (e.g.,typically silver) ions into or out of the conducting channel duringsubsequent programming and erasing forms or dissolves a conductivepathway along the conducting channel, which causes a detectibleconductivity (or resistance) change across the memory device.

It has been determined that Ge₄₀Se₆₀ works well as the glass backbone ina PCRAM device because this stoichiometry makes for a glass that isrigid and incorporates thermodynamically unstable germanium-germanium(Ge—Ge) bonds. The presence of another species, such as silver selenideprovided from an accompanying layer, can, in the presence of an appliedpotential, break the Ge—Ge bonds and bond with the previously homopolarbonded Ge to form conducting channels. These characteristics make this“40/60” stoichiometry optimal when using a germanium selenidechalcogenide glass with respect to the formation of a conducting channeland conductive pathway.

While germanium-chalcogenide (e.g., Ge₄₀Se₆₀) glass layers are highlydesirable for PCRAM devices, other glasses may be desirable to improveswitching properties or thermal limitations of the devices.

SUMMARY

The invention provides embodiments of a method of determining suitableglass backbone material, which may be used in place of Ge₄₀Se₆₀ glass ina resistance variable memory device, such as a PCRAM, with othermaterials, a method of forming memory devices with such materials, anddevices constructed in accordance with these methods.

The chalcogenide glass material may be represented by A_(x)B_(100-x),where A is a non-chalcogenide material selected from Groups 3-15 of theperiodic table and B is a chalcogenide material from Group 16. Themethod of selecting a glass material includes: (1) selection of anon-chalcogenide component A from Groups 3-15 that will exhibithomopolar bonds; (2) selection of a chalcogenide component B from Group16 for which component A will have a bonding affinity, relative to theA-A homopolar bonds; (3) selection of a stoichiometry (i.e., x ofA_(x)B_(100-x)) that will allow the homopolar A-A bonds to form; and (4)confirmation that the glass A_(x)B_(100-x), at the selectedstoichiometry (i.e., x), will allow a conducting channel and aconductive pathway to form therein upon application of a conditioningvoltage (when a metal-chalcogenide layer and metal ions are proximatethe glass).

An exemplary memory device constructed in accordance with an embodimentof the invention uses a germanium telluride glass backbone having aGe_(x)Te_(100-x) stoichiometry and a metal-chalcogenide layer proximatethereto for a memory cell. In a specific exemplary embodiment, x isbetween about 44 and about 53. Also, the metal-chalcogenide layer can bea tin selenide with a stoichiometry of about SnSe. Other layers may alsobe associated with this glass backbone and metal-chalcogenide layer.

The above and other features and advantages of the invention will bebetter understood from the following detailed description, which isprovided in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 show graphs of Raman shift analysis of germanium selenideglass, which may be used in selecting glass backbone materials inaccordance with the invention;

FIG. 4 shows an exemplary embodiment of a memory device in accordancewith the invention;

FIG. 5 shows an exemplary embodiment of a memory device in accordancewith the invention;

FIGS. 6-11 show a cross-section of a wafer at various stages during thefabrication of a device in accordance with an embodiment of theinvention;

FIG. 12 shows a resistance-voltage curve of a first (conditioning) writeand second (programming) write for a 0.13 μm device in accordance withthe invention;

FIG. 13 shows an exemplary processor-based system incorporating memorydevices in accordance with the invention;

FIGS. 14 a-14 h are graphs showing experimental results of thermaltesting conducted with devices fabricated in accordance with exemplaryembodiments of the invention; and

FIG. 15 shows a graph of Raman shift analysis of germanium tellurideglass.

DETAILED DESCRIPTION

In the following detailed description, reference is made to variousspecific embodiments of the invention. These embodiments are describedwith sufficient detail to enable those skilled in the art to practicethe invention. It is to be understood that other embodiments may beemployed, and that various structural, logical and electrical changesmay be made without departing from the spirit or scope of the invention.

The term “substrate” used in the following description may include anysupporting structure including, but not limited to, a semiconductorsubstrate that has an exposed substrate surface. A semiconductorsubstrate should be understood to include silicon-on-insulator (SOI),silicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxiallayers of silicon supported by a base semiconductor foundation, andother semiconductor structures. When reference is made to asemiconductor substrate or wafer in the following description, previousprocess steps may have been utilized to form regions or junctions in orover the base semiconductor or foundation. The substrate need not besemiconductor-based, but may be any support structure suitable forsupporting an integrated circuit, including, but not limited to, metals,alloys, glasses, polymers, ceramics, and any other supportive materialsas is known in the art.

The term “chalcogenide” is intended to include various alloys,compounds, and mixtures of chalcogens (elements from Group 16 of theperiodic table, e.g., sulfur (S), selenium (Se), tellurium (Te),polonium (Po), and oxygen (O)).

Embodiments of the invention provide a method of selecting a glassbackbone material for use in a resistance variable memory device, suchas a PCRAM. The backbone material (i.e., backbone glass layer 18 ofFIGS. 4 and 5) may be represented by the formula A_(x)B_(100-x), where Ais a non-chalcogenide material selected from Groups 3-15, and preferably13, 14, and 15, of the periodic table and B is a chalcogenide material.The glass backbone may also be represented by the formula(A_(x)B_(100-x))C_(y), where C represents one or more additional,optional components, which may be present in some glass formulations,but which may be omitted; therefore, the description hereafter willfocus on two components (A and B) for simplicity sake. The ultimatechoice for the material A_(x)B_(100-x) depends, in part, on the make upof an adjacent metal-chalcogenide layer (e.g., layer 20 of FIGS. 4 and5) with which it operationally engages. The component A should have anaffinity for the chalcogenide component B, and preferably for thechalcogenide material (which is preferably also component B) of themetal-chalcogenide layer. Since Ge₄₀Se₆₀glass has been experimentallyobserved to have good glass backbone properties in PCRAM devices, abackbone material represented by the formula A_(x)B_(100-x) should haveproperties similar to Ge₄₀Se₆₀ glass, such as homopolar bond propertiesand affinity of the non-chalcogenide component, e.g., Ge, for thechalcogenide component in the metal-chalcogenide layer.

Taking these characteristics into consideration, a primary considerationin selecting components A and B and the stoichiometry for a glassbackbone material is that the resulting material containthermodynamically unstable homopolar bonds of component A, meaning thatthe non-chalcogenide component A may form a bond with another componentA in the glass, as initially formed, only if there is an insufficientamount of component B to satisfy the coordination number requirement forcomponent A, which allows for the formation homopolar A-A bonds. The A-Ahomopolar bonds in such a glass material are thermodynamically unstableand will themselves break when the device is programmed and a conductingchannel is formed in the glass backbone by the metal-chalcogenide layerwhen the chalcogenide component for the metal-chalcogenide layer bondsto the component A participating in the homopolar bonds. This propertyis dependent on the stoichiometry of the material A_(x)B_(100-x) in thatexcess chalcogenide component B will inhibit the formation of homopolarA-A bonds.

What “excess” chalcogenide component B means in relation to the materialA_(x)B_(100-x) and its stoichiometry may be determined by whether thematerial exhibits the homopolar bonds or not. Raman spectroscopy can bea useful analytical tool for determining the presence of homopolar bondswhen selecting a material A_(x)B_(100-x) for the glass backbone in PCRAMdevices. Raman Spectroscopy is based on the Raman effect, which is theinelastic scattering of photons by molecules. A plot of Raman intensity(counts) vs. Raman shift (cm⁻¹) is a Raman spectrum, which is the basisfor FIGS. 1-3.

Referring now to FIG. 1, which is a Raman spectrum for bulk Ge₂₃Se₇₇glass, it can be observed by the Raman Shift peaks at about 200 cm⁻¹ andat about 260 cm⁻¹ that the glass incorporates Ge—Se bonds and Se—Se(i.e., chalcogenide) bonds. This is an undesirable stoichiometry for theglass backbone since it lacks the homopolar Ge—Ge (i.e.,non-chalcogenide) bonds desirable for switching in a device comprising agermanium selenide glass. Compare FIG. 1 to FIG. 2, the latter of whichis a Raman spectrum for bulk Ge₄₀Se₆₀ glass, which shows a Raman Shiftpeak at about 175 cm⁻¹ that corresponds to Ge—Ge homopolar bonds and apeak at about 200 cm⁻¹ that corresponds to Ge—Se bonds. The prevalenceof non-chalcogenide (i.e., Ge—Ge) homopolar bonds found in Ge40Se60 is acharacteristic sought in materials for the glass backbone in PCRAM. Thischaracteristic may also be seen in a thin film of Ge₄₀Se₆₀ using Ramanspectra, as shown in FIG. 3. Similar comparisons of Raman spectra may bemade for other materials A_(x)B_(100-x) of varying stoichiometries tofind spectra showing peaks demonstrating homopolar bonding of thenon-chalcogenide component (i.e., A-A) in the material, which indicatesthat it has suitable properties for a glass backbone.

Taking the aforementioned desired characteristics into consideration,the method of detecting a suitable glass backbone may be performed bythe following steps: (1) selection of a non-chalcogenide component Afrom Groups 3-15 that will exhibit homopolar bonds; (2) selection of achalcogenide component B from Group 16 for which component A will have abonding affinity, relative to the A-A homopolar bonds; (3) selection ofa stoichiometry (i.e., x of A_(x)B_(100-x)) that will provide for athermodynamically unstable glass and will allow the homopolar A-A bondsto form; and (4) confirmation that the glass A_(x)B_(100-x), at theselected stoichiometry (i.e., x), will allow a conducting channel and aconductive pathway to form therein upon application of a conditioningvoltage (when a metal-chalcogenide, e.g., M_(y)B_(100-y), and metal ionsare proximate the glass).

Using the above-discussed methodology for selecting glass backbonematerials A_(x)B_(100-x), at least four have been found preferable foruse in PCRAM devices. These materials include arsenic selenide,represented by formula As₅₀Se₅₀, tin selenide, represented by formulaSn₅₀Se₅₀, antimony selenide, represented by formula Sb_(x)Se_(100-x),and germanium telluride, represented by formula Ge_(x)Te_(100-x). Asshown by FIG. 15, the Raman shift peaks for germanium telluride glassare at about 140 counts/cm⁻¹ (for Ge—Ge bonds) and about 180 counts/cm⁻¹(for Te—Te bonds), demonstrating at least one favorable characteristicof the glass for PCRAM. Although each of these exemplary materialsinclude selenium or tellurium for component B, other chalcogenides maybe used as well.

The invention is now explained with reference to the other figures,which illustrate exemplary embodiments and throughout which likereference numbers indicate like features. FIG. 4 shows an exemplaryembodiment of a memory device 100 constructed in accordance with theinvention. The device 100 shown in FIG. 4 is supported by a substrate10. Over the substrate 10, though not necessarily directly so, is aconductive address line 12, which serves as an interconnect for thedevice 100 shown and for a plurality of other similar devices of aportion of a memory array of which the shown device 100 is a part. It ispossible to incorporate an optional insulating layer (not shown) betweenthe substrate 10 and address line 12, and this may be preferred if thesubstrate 10 is semiconductor-based. The conductive address line 12 canbe any material known in the art as being useful for providing aninterconnect line, such as doped polysilicon, silver (Ag), gold (Au),copper (Cu), tungsten (W), nickel (Ni), aluminum (Al), platinum (Pt),titanium (Ti), and other materials.

Over the address line 12 is a first electrode 16, which can be definedwithin an insulating layer 14 (or may be a common blanket electrodelayer; not shown), which is also over the address line 12. Thiselectrode 16 can be any conductive material that will not migrate intochalcogenide glass, but is preferably tungsten (W). The insulating layer14 should not allow the migration of metal ions and can be an insulatingnitride, such as silicon nitride (Si₃N₄), a low dielectric constantmaterial, an insulating glass, or an insulating polymer, but is notlimited to such materials.

A memory element, i.e., the portion of the memory device 100 whichstores information, is formed over the first electrode 16. In theembodiment shown in FIG. 4, a chalcogenide glass layer 18 is providedover the first electrode 16. The chalcogenide glass layer 18 has thestoichiometric formula A_(x)B_(100-x), with A being a non-chalcogenidecomponent and B being a chalcogenide component as discussed above. Thematerial A_(x)B_(100-x) may be many materials with the appropriatecharacteristics (e.g., homopolar bonds, homopolar bond strength,thermodynamic instability, etc.) and an appropriate stoichiometry, asdiscussed above, but is preferably be selected from Sn₅₀Se₅₀,Sb_(x)Se_(100-x), As₅₀Se₅₀, and Ge_(x)Te_(100-x), which have been foundto be suitable by following the methodology discussed above. Germaniumtelluride with a formula Ge_(x)Te_(100-x), where x is between about 44and 53, is the preferred material for layer 18. More preferably, x isbetween 46 and 51 and most preferably, x is about 47. This may bewritten as Ge₄₆Te₅₄ to Ge₅₁Te₄₉. Germanium telluride is a particularlygood selection for the chalcogenide glass layer 18 because, as shown bythe Raman data in FIG. 15, it exhibits the Ge—Ge homopolar bondsdesirable for such glass. Also, when used with a metal-chalcogenidelayer 20 of tin selenide (SnSe), the tin selenide allows for theformation of a Ge—Se bond and the development of a channel for metal(e.g., Ag) ion migration during operation of the memory device.

The layer of chalcogenide glass 18 is preferably between about 100 Å andabout 1000 Å thick, most preferably about 300 Å thick. Layer 18 need notbe a single layer of glass, but may also be comprised of multiplesub-layers of chalcogenide glass having the same or differentstoichiometries. This layer of chalcogenide glass 18 is in electricalcontact with the underlying electrode 16.

Over the chalcogenide glass layer 18 is a layer of metal-chalcogenide20, which may be any combination of metal component M, which may beselected from any metals, and chalcogenide component B, which ispreferably the same chalcogenide as in the glass backbone layer 18, andmay be represented by the formula M_(y)B_(100-y). As with the glassbackbone layer 18 material, other components may be added, but themetal-chalcogenide will be discussed as only two components M and B forsimplicity sake. The metal-chalcogenide may be, for example, silverselenide (Ag_(y)Se, y being about 20) or, preferably, tin selenide(Sn_(10+/−y)Se, where y is between about 10 and 0). Themetal-chalcogenide layer 20 is preferably about 500 Å thick; however,its thickness depends, in part, on the thickness of the underlyingchalcogenide glass layer 18. The ratio of the thickness of themetal-chalcogenide layer 20 to that of the underlying chalcogenide glasslayer 18 should be between about 5:1 and about 1:1, more preferablyabout 2.5:1.

Still referring to FIG. 4, a metal layer 22 is provided over themetal-chalcogenide layer 20, with the metal of layer 22 preferablyincorporating some silver, if not being exclusively silver. The metallayer 22 should be about 500 Å thick. The metal layer 22 assists theswitching operation of the memory device 100. Over the metal layer 22 isa second electrode 24. The second electrode 24 can be made of the samematerial as the first electrode 16, but is not required to be so. In theexemplary embodiment shown in FIG. 4, the second electrode 24 ispreferably tungsten (W). The device(s) 100 may be isolated by aninsulating layer 26.

FIG. 5 shows another exemplary embodiment of a memory device 101constructed in accordance with the invention. Memory device 101 has manysimilarities to memory device 100 of FIG. 4 and layers designated withlike reference numbers are preferably the same materials and have thesame thicknesses as those described in relation to the embodiment shownin FIG. 4. For example, the first electrode 16 is preferably tungsten.The chalcogenide glass layer 18 material A_(x)B_(100-x) is selectedaccording to the methodology detailed above and can be germaniumtelluride; it is preferably about 150 Å thick. As with device 100 ofFIG. 4, the metal-chalcogenide layer 20 may be any combinationM_(y)B_(100-y), but can be tin selenide; it is preferably about 470 Åthick. The metal layer 22 preferably contains some silver, but can bemostly or wholly silver; it is preferably about 200 Å thick. The primarydifference between device 100 and device 101 is the addition to device101 of additional second and third chalcogenide layers 18 a and 18 b.

The second chalcogenide glass layer 18 a is formed over themetal-chalcogenide layer 20 and is preferably about 150 Å thick. Overthis second chalcogenide glass layer 18 a is metal layer 22. Over themetal layer 22 is a third chalcogenide glass layer 18 b, which ispreferably about 100 Å thick. The third chalcogenide glass layer 18 bprovides an adhesion layer for subsequent electrode formation. As withlayer 18 of FIG. 4, layers 18 a and 18 b are not necessarily a singlelayer, but may be comprised of multiple sub-layers. Additionally, thesecond and third chalcogenide layers 18 a and 18 b may be a differentglass material from the first chalcogenide glass layer 18 or from eachother. Glass material preferred for layers 18 a and 18 b is germaniumselenide (Ge_(x)Se_(100-x)), more preferably Ge₂Se₃, but other materialsmay be useful as well, including germanium telluride (Ge_(x)Te_(100-x)),arsenic selenide (As_(x)Se_(100-x)), tin selenide (Sn_(x)Se_(100-x)),antimony selenide (Sb_(x)Se_(100-x)), germanium sulfide(Ge_(x)Se_(100-x)), and combinations of germanium (Ge), silver (Ag), andselenium (Se). The second electrode 24 is preferably tungsten (W), butmay be other metals also.

As shown by FIGS. 14 a-14 h, PCRAM devices in accordance with theabove-discussed embodiment (FIG. 5) were experimentally tested formemory operation under varied thermal conditions. Each chart (FIGS. 14a-14 h) represents a set of thermal tests on a respective PCRAM device.Similar to the device shown in FIG. 5, each tested device had a tungsten(W) first electrode (e.g., layer 16), a 300 Å germanium telluride(Ge_(x)Te_(100-x), x≅44 to 53) layer (e.g., layer 18) thereover, a 900 Åtin selenide (SnSe) layer (e.g., layer 20) thereover, a 150 Å germaniumselenide (Ge₂Se₃) layer (e.g., layer 18 a) thereover, a 500 Å silver(Ag) layer (e.g., layer 22) thereover, a 100 Å germanium selenide(Ge₂Se₃) layer (e.g., layer 18 b) thereover, and a tungsten (W) secondelectrode (e.g., layer 24).

The testing was conducted by positioning wafers, which supported thePCRAM devices, on a temperature-controllable chuck and DC programmingten (10) devices at the temperatures shown on the charts in FIGS. 14a-14 h. The DC probing procedure was conducted on each device asfollows: (1) sweep potential from 0 to 800 mV and read resistance ofcell at 10 mV to determine the initial resistance (Ri); (2) sweep thepotential from 0 to 10 mV and record the resistance at 10 mV to obtainthe write resistance (Rw1); (3) sweep the device from 0 to −1 V andrecord the potential at which the device erased and the current at thaterase potential to determine the erase voltage and erase current; (4)sweep the device from 0 to 800 mV and read the resistance at 10 mV (thisis the Rerase) and record the potential at which the device switched(i.e., was written; the Vw2); and (5) sweep the potential from 0 to 10mV and record the resistance at 10 mV (this is the Rw2). The charts ofFIGS. 14 a-14 h show these measured parameters (i.e., Vw1, Vw2, Ri,erase current, erase voltage, Rw1, Rw2, Rerase) for 10 experimentaldevices constructed in accordance with the invention at the temperaturesshown along the x-axis of the charts. The results show that germaniumtellurium based PCRAM cells have thermal tolerances suitable for use inmemory devices.

The above-discussed embodiments are exemplary embodiments of theinvention; however, other exemplary embodiments may be used whichcombine the first electrode layer 16 and address line layer 12. Anotherexemplary embodiment may use blanket layers (e.g., layers 16, 18, 20,and 22 of FIG. 4) of the memory cell body, where the memory cell isdefined locally by the position of the second electrode 24 over thesubstrate 10. Another exemplary embodiment may form the memory devicewithin a via. Additional layers, such as barrier layers oralloy-controlling layers, not specifically disclosed in the embodimentsshown and discussed above, may be added to the devices in accordancewith the invention without departing from the scope thereof.

FIGS. 6-11 illustrate a cross-sectional view of a wafer during thefabrication of a memory device 100 as shown by FIG. 4. Although theprocessing steps shown in FIGS. 6-11 most specifically refer to memorydevice 100 of FIG. 4, the methods and techniques discussed may also beused to fabricate memory devices of other embodiments (e.g., device 101of FIG. 5) as would be understood by a person of ordinary skill in theart.

As shown by FIG. 6, a substrate 10 is provided. As indicated above, thesubstrate 10 can be semiconductor-based or another material useful as asupporting structure as is known in the art. If desired, an optionalinsulating layer (not shown) may be formed over the substrate 10; theoptional insulating layer may be silicon nitride or other insulatingmaterials used in the art. Over the substrate 10 (or optional insulatinglayer, if desired), a conductive address line 12 is formed by depositinga conductive material, such as doped polysilicon, aluminum, platinum,silver, gold, nickel, but preferably tungsten, patterning one or moreconductive lines, for instance with photolithographic techniques, andetching to define the address line 12. The conductive material may bedeposited by any technique known in the art, such as sputtering,chemical vapor deposition, plasma enhanced chemical vapor deposition,evaporation, or plating.

Still referring to FIG. 6, over the address line 12 is formed aninsulating layer 14. This layer 14 can be silicon nitride, a lowdielectric constant material, or many other insulators known in the artthat do not allow metal (e.g., silver, copper, or other metal) ionmigration, and may be deposited by any method known in the art. Anopening 14 a in the insulating layer is made, for example, byphotolithographic and etching techniques, thereby exposing a portion ofthe underlying address line 12. Over the insulating layer 14, within theopening 14 a, and over the address line 12 is formed a conductivematerial, preferably tungsten (W). A chemical mechanical polishing stepmay then be utilized to remove the conductive material from over theinsulating layer 14, to leave it as a first electrode 16 over theaddress line 12, and planarize the wafer.

FIG. 7 shows the cross-section of the wafer of FIG. 6 at a subsequentstage of processing. A series of layers making up the memory device 100(FIG. 4) are blanket-deposited over the wafer. A chalcogenide glasslayer 18 is formed to a preferred thickness of about 300 Å over thefirst electrode 16 and insulating layer 14. The chalcogenide glass layer18 is Ge_(x)Te_(100-x), where x is between about 44 to 53, but may alsobe selected from other materials such as As₅₀Se₅₀, Sn₅₀Se₅₀, andSb_(x)Se_(100-x), and as described above, may be selected from manymaterials A_(x)B_(100-x) with appropriate characteristics and of anappropriate stoichiometry for memory function.

The steps in selecting a chalcogenide glass layer 18 material are: (1)selection of a non-chalcogenide component A from Groups 3-15 that willexhibit homopolar bonds; (2) selection of a chalcogenide component Bfrom Group 16 for which component A will have a bonding affinity,relative to the A-A homopolar bonds; (3) selection of a stoichiometry(i.e., x of A_(x)B_(100-x)) that will provide for thermodynamicallyunstable homopolar A-A bonds; and (4) confirmation that the glassA_(x)B_(100-x), at the selected stoichiometry (i.e., x), will allow aconducting channel and a conductive pathway to form therein uponapplication of a conditioning voltage when a metal-chalcogenide layer 20is proximate the glass layer 18. Once the materials are selected,deposition of the chalcogenide glass layer 18 may be accomplished by anysuitable method, such as evaporative techniques or chemical vapordeposition; however, the preferred technique utilizes either sputteringor co-sputtering.

Still referring to FIG. 7, a metal-chalcogenide layer 20, e.g.,M_(y)B_(100-y), is formed over the chalcogenide glass layer 18. Themetal-chalcogenide layer 20 is preferably tin selenide (SnSe),particularly when germanium telluride is used as the chalcogenide glasslayer 18. Physical vapor deposition, chemical vapor deposition,co-evaporation, sputtering, or other techniques known in the art may beused to deposit layer 20 to a preferred thickness of about 500 Å. Again,the thickness of layer 20 is selected based, in part, on the thicknessof layer 18 and the ratio of the thickness of the metal-chalcogenidelayer 20 to that of the underlying chalcogenide glass layer 18 ispreferably from about 5:1 to about 1:1, more preferably about 2.5:1. Itshould be noted that, as the processing steps outlined in relation toFIGS. 6-11 may be adapted for the formation of other devices inaccordance the invention, e.g., the layers may remain inblanket-deposited form, a barrier or alloy-control layer may be formedadjacent to the metal-chalcogenide layer 20, on either side thereof, orthe layers may be formed within a via.

Still referring to FIG. 7, a metal layer 22 is formed over themetal-chalcogenide layer 20. The metal layer 22 preferably incorporatesat least some silver (Ag), if not exclusively being silver (Ag), but maybe other metals as well, such as copper (Cu) or a transition metal, andis formed to a preferred thickness of about 300 Å. The metal layer 22may be deposited by any technique known in the art.

Still referring to FIG. 7, over the metal layer 22, a conductivematerial is deposited for a second electrode 24. Again, this conductivematerial may be any material suitable for a conductive electrode, but ispreferably tungsten; however, other materials may be used such astitanium nitride or tantalum, for example.

Now referring to FIG. 8, a layer of photoresist 28 is deposited over thetop electrode 24 layer, masked and patterned to define the stacks forthe memory device 100, which is but one of a plurality of like memorydevices of a memory array. An etching step is used to remove portions oflayers 18, 20, 22, and 24, with the insulating layer 14 used as an etchstop, leaving stacks as shown in FIG. 9. The photoresist 28 is removed,leaving a substantially complete memory device 100, as shown by FIG. 9.An insulating layer 26 may be formed over the device 100 to achieve astructure as shown by FIGS. 4, 10, and 11. This isolation step can befollowed by the forming of connections (not shown) to other circuitry ofthe integrated circuit (e.g., logic circuitry, sense amplifiers, etc.)of which the memory device 100 is a part, as is known in the art.

As shown in FIG. 10, a conditioning step is performed by applying avoltage pulse of about 0.20 V to incorporate material from themetal-chalcogenide layer 20 into the chalcogenide glass layer 18 to forma conducting channel 30 in the chalcogenide glass layer 18. Theconducting channel 30 will support a conductive pathway 32, as shown inFIG. 11, upon application of a programming pulse of about 0.17 V duringoperation of the memory device 100.

The embodiments described above refer to the formation of only a fewpossible resistance variable memory device structures (e.g., PCRAM) inaccordance with the invention, which may be part of a memory array. Itmust be understood, however, that the invention contemplates theformation of other memory structures within the spirit of the invention,which can be fabricated as a memory array and operated with memoryelement access circuits.

FIG. 12 shows a resistance-voltage curve of a first write, whichcorresponds to a conditioning voltage, and a second write, whichcorresponds to a programming voltage, for a 0.13 μm device such asdevice 100 or 101 shown in FIGS. 4 and 5, respectively. The devicerepresented by the curve of FIG. 12 has an As₅₀Se₅₀ chalcogenide glasslayer 18 (See FIGS. 4 and 5). FIG. 12 shows that the first write is at aslightly higher potential than the second write (i.e., about 0.2 V.compared to about 0.17 V, respectively). This is because the first writeconditions the device in accordance with the processing shown at FIG. 10by forming a conducting channel 30, which remains intact after thisfirst write. The second write requires less voltage because the stableconducting channel 30 is already formed by the conditioning write andthe conductive pathway 32 is formed more easily. Application of thesewrite voltages programs the device to a non-volatile higherconductivity, lower resistivity, memory state. These observedprogramming parameters utilizing a chalcogenide glass layer 18 selectedin accordance with the invention show that the tested devices work aswell as when Ge₄₀Se₆₀ is used as the glass backbone.

The erase potential for a device having an As₅₀Se₅₀ chalcogenide glasslayer (e.g., layer 18) is also similar to a device having Ge₄₀Se₆₀glass. This erase voltage curve is not shown in FIG. 12; however, theerase potential is about −0.06 V, which returns the device to anon-volatile higher resistance, lower conductivity memory state.

FIG. 13 illustrates a typical processor system 400 which includes amemory circuit 448, e.g., a PCRAM device, which employs resistancevariable memory devices (e.g., devices 100 and 101) fabricated inaccordance with an embodiment the invention. A processor system, such asa computer system, generally comprises a central processing unit (CPU)444, such as a microprocessor, a digital signal processor, or otherprogrammable digital logic devices, which communicates with aninput/output (I/O) device 446 over a bus 452. The memory circuit 448communicates with the CPU 444 over bus 452 typically through a memorycontroller.

In the case of a computer system, the processor system may includeperipheral devices such as a floppy disk drive 454 and a compact disc(CD) ROM drive 456, which also communicate with CPU 444 over the bus452. Memory circuit 448 is preferably constructed as an integratedcircuit, which includes one or more resistance variable memory devices,e.g., device 100. If desired, the memory circuit 448 may be combinedwith the processor, for example CPU 444, in a single integrated circuit.

The above description and drawings should only be consideredillustrative of exemplary embodiments that achieve the features andadvantages of the invention. Modification and substitutions to specificprocess conditions and structures can be made without departing from thespirit and scope of the invention. Accordingly, the invention is not tobe considered as being limited by the foregoing description anddrawings, but is only limited by the scope of the appended claims.

1. A method of forming a memory device, comprising: providing a firstelectrode and a second electrode; providing a metal-chalcogenide layerbetween said first and second electrodes, wherein saidmetal-chalcogenide layer comprises tin selenide; and providing agermanium telluride glass between said first and second electrodes andin contact with said metal-chalcogenide layer.
 2. The method of claim 1,wherein said metal-chalcogenide layer is between said chalcogenide glasslayer and said second electrode.
 3. The method of claim 1, wherein saidmetal-chalcogenide layer comprises silver selenide.
 4. The method ofclaim 1, wherein said germanium telluride layer has a stoichiometricformula Ge_(x)Te_(100-x), where x is about 44 to about
 53. 5. The methodof claim 1, wherein said germanium telluride layer has a stoichiometricformula Ge_(x)Te_(100-x), where x is about 46 to about
 51. 6. The methodof claim 1, wherein said germanium telluride layer has a stoichiometricformula Ge_(x)Te_(100-x), where x is about
 47. 7. The method of claim 1,further comprising providing a metal layer between saidmetal-chalcogenide layer and said second electrode.
 8. The method ofclaim 7, wherein said metal layer comprises silver.
 9. The method ofclaim 7, further comprising providing a first chalcogenide glass layerbetween said metal-chalcogenide layer and said metal layer.
 10. Themethod of claim 9, further comprising providing a second chalcogenideglass layer between said metal layer and said second electrode.